1. Field of the Invention
The present invention relates generally to electrodes for use in electrochemical devices, and more particularly, to composite electrodes and encapsulated electrode particles for use in solid electrochemical devices.
2. Description of the Related Art
Solid electrochemical devices such as the lithium solid polymer electrolyte rechargeable battery is an attractive technology for rechargeable battery applications due to its high predicted energy density, freedom in battery configuration, minimal potential for environmental and safety hazard, and low associated materials and processing costs. The solid electrochemical device usually consists of a solid electrolyte interposed between an anode and a cathode. The solid electrolyte may be a composite consisting of a solid polymeric matrix and an inorganic ion salt. In some implementations, it may consist also of an electrolyte solvent, which acts as a platicizer of the polymer. The cathode may also be a composite material composed of: a solid polymeric matrix, an electroactive material (e.g., a lithium compound), and solvents. The anode is typically an electroactive material capable of intercalating lithium, such as carbon or a metal.
A solid lithium battery is charged by applying a voltage between the battery's electrodes (e.g. cathode and anode networks), which causes lithium ions and electrons to be withdrawn from lithium hosts at the battery's cathode. Lithium ions flow from the cathode to the battery's anode through a polymer electrolyte to be reduced at the anode, the overall process requiring energy. Upon discharge, the reverse occurs; lithium ions and electrons are allowed to re-enter lithium hosts at the cathode while lithium is oxidized to lithium ions at the anode, an energetically favorable process that drives electrons through an external circuit, thereby supplying electric power to a device to which the battery is connected.
For most lithium ion batteries, power density (i.e., the power available from the battery based upon its weight (W/kg) or volume (W/l)) is limited by the resistance to the movement of ions or electrons through the cathode network, the separator and the anode network. This transport-limited quantity is currently a major limitation to the widespread use of lithium ion technology. In order for the lithium ion-electron exchange reactions to occur, it is particularly important that the electroactive materials of the electrodes come into direct contact with the electrolyte material. To facilitate such contact, the common approach has been to maintain an interconnected network, which is filled with a liquid electrolyte or a polymeric gel (within which a liquid electrolyte is dissolved). To assure sufficient levels of electronic conductivity, the particles of electroactive material of the electrodes are usually mixed with fine particles of an electrically conductive material such as graphite or carbon black.
For example, cathodes in some rechargeable lithium batteries contain lithium ion host materials, electrically conductive particles that electrically connect the lithium ion hosts to each other and a current collector (i.e., a battery terminal), a binder, and a lithium-conducting liquid electrolyte. The lithium ion hosts typically are particles of lithium intercalation compounds; the electrically conductive particles are typically made of a substance such as carbon black or graphite. The resulting cathode includes a mixture of particles of average size usually on the order of no more than about 20 micron. Anodes for rechargeable lithium-ion batteries typically contain a lithium ion host material such as graphite, optionally a conductive additive, a binder, and a lithium ion-conducting liquid electrolyte.
Generally, when producing these electrodes, a suspension is made of a cathode oxide powder, a binder (e.g., poly(vinylidene fluoride))(“PVDF”), a fine conductive additive (e.g., high surface area conductive carbon), and a solvent to produce a castable suspension. The film is cast, printed, or coated on a metal foil current collector or an insulating film. After drying, the electrode is laminated with a separator and a counterelectrode and infused with organic liquid electrolyte to produce a battery cell. Optionally, a cathode oxide with high electronic conductivity, such as LiMg0.05Co0.95O2, is used and no carbon additive is used. However, the power density of such a cell would be quite low.
One drawback with using carbon-based materials as the electrically conductive material is that it significantly alters the rheology of the slurry. Small variations in amount of carbon lead to large variations in coating quality and product performance, resulting in an improper distribution of the electric potential within the electroactive material or incomplete connection of electroactive material to the current collector. Further, variations in the local concentration of mobile ions and gradients within the electroactive material may impair the reversibility of the electrode to operate over a large number of cycles. At a microscopic level, these non-uniform stresses may result in the disintegration of particles of the electroactive material, or the loss of contact between these particles with the surrounding electrically conductive carbon material. These effects may be amplified when significant current density and power are required at the electrode.
An electrode system with higher power and energy densities than the present batteries provide is needed. Energy density (i.e., the ratio of the energy available from the cell to its volume (Wh/l) or weight (Wh/kg)) can be intrinsically determined by the storage materials; the cell voltage can be determined by the difference in lithium electrochemical potential between cathode and anode, while the charge capacity can depend on the lithium concentration that can be reversibly intercalated by the cathode and anode. Power density, on the other hand, can be a transport-limited quantity, determined by the rate at which electrons and lithium ions can be inserted into or removed from the electrodes.
An electrode in a lithium battery that is too thick can limit discharge rate because ion transport from the electrode to the interface with a separator, such as the electrolyte, can be rate limiting. On the other hand, if the electrode layers are very thin, then energy density suffers because the electrolyte, separator, and current collectors occupy a higher volume and contribute to a greater mass relative to the active material of the electrodes. In addition, the discharge rate can be limited by interface resistance that allows only a certain current rate per unit area of interface.
It has been suggested that a conductive polymer may be used to replace carbon as the electrically conductive material in composite electrodes with the hope that the electronic conductivity of the electrode, particularly the cathode, may be improved with a lightweight substitute material. However, it has been found that the electronic conductivity of these materials is generally too low to assure high power density for the battery. Further, these prior art materials have been found to lack electrochemical stability, and thus, are not amenable for use in high cycle long-life secondary cells.
Shackle et al., U.S. Pat. No. 6,174,623, teach coating vanadium oxide electroactive particles with a polyaniline conductive polymer. However, it has been reported that polyaniline compounds may lack the desired level of electrochemical stability such that the cycle life of the constructed battery is undesirably short. This drawback is indicated by charge-discharge tests of polyaniline materials, which are reported in Kuwabata et al., Electrochemical Acta, 44:4593–4600 (1999). Another disadvantage of the use of doped polyaniline in place of carbon as a conductive material its low conductivity. For example, it is well known in the art that an acid dopant is required to render virgin polyaniline conductive. Cao et al., U.S. Pat. No. 5,624,605, teach that an acid dopant with a long alkyl group can render polyaniline soluble and, therefore, processable. Shackle teaches the use of such a functional dopant to render the conducting polyaniline soluble and processable, to enable coating of the cathode particles. Cao et al. teach that polyaniline doped in such a way can have a maximum conductivity of about 2 S/cm (a 90:10 blend with polystyrene). The conductivity can be higher (up to about 10 S/cm), if the doped polyaniline is mixed with polyethylene and stretched to orient the conductive domains. However, this technique is not possible when applying the conductive composition to the cathode particle surface by spray drying as taught by Shackle.
Thus, there is significant room for improvements for conductive materials for composite electrodes that provides a combination of sufficient electronic and ionic conductivity, and electrochemical stability to produce batteries with long cycle life and high power density.